JOURNAL OF THE EXPERIMENTAL ANALYSIS OF BEHAVIOR

COMPOUNDING DISCRIMINATIVE STIMULICONTROLLING FREE-OPERANT AVOIDANCE"

HENRY H. EMURIAN AND STANLEY J. WEISS

THE AMERICAN UNIVERSITY

The performances of three rats were stabilized on a multiple schedule that maintainedresponding by a free-operant avoidance schedule during independent presentations oftone and light. The simultaneous absence of these stimuli signalled shock-free periods andcontrolled response cessation. Subsequently, test sessions were administered consisting ofindependent presentations of each stimulus and these stimuli compounded (tone-plus-light). During an extinction test, additive summation was observed to the compoundedstimuli, i.e., more responses were emitted to the compound than to either tone or light.During a series of 28 maintenance-test sessions in which the shock schedule remainedoperative, the compounded stimuli produced a generally enhanced response rate andfewer pauses terminating with shock than either single stimulus condition. These resultsextend the generality of free-operant additive summation to responding maintained byaversive control. In addition, a comparison of the present study with previous experimentsreporting additive summation of positively reinforced responding indicates that similarvariables-rate and aversive differences between training stimulus conditions-should beconsidered in accounting for response distributions during stimulus compounding whenresponding is controlled by either positive or negative contingencies.

Additive summation of free-operant re-sponding is observed when compound pre-sentations of independently conditioned dis-criminative stimuli produce a greater responserate than either stimulus presented separately.Additive summation has been demonstratedduring extinction compounding tests whenresponding during the independently condi-tioned stimuli was maintained in training byvariable-interval (VI) schedules (Cornell andStrub, 1965; Weiss, 1964, 1969, 1971 Experi-ment 2; Wolf, 1963 Experiment 1) or by differ-ential-reinforcement-of-low-rate (DRL) sched-ules (Weiss, 1968), and during compoundingtests where reinforcement was continued whenresponding during the independent stimuliwas maintained in training by VI (Meltzer andFreeman, 1971; Wolf, 1963 Experiment 2) and

'This experiment was based on a thesis submitted tothe department of Psychology, The American Univer-sity, in partial fulfillment of the requirements for theM. A. degree. It was supported by Grant MH-16853 tothe second author from the National Institute ofMental Health, United States Public Health Service.An earlier version of this paper was presented at the1970 American Psychological Association Conventionin Miami. The authors thank Maureen Christian,Larry Cohen, and Susan Levine for their assistance.Reprints may be obtained from Henry H. Emurian,The American University, Department of Psychology,Washington, D. C. 20016.

by fixed-interval schedules (Miller and Ackley,1970).Whereas the consistency of these results

established free-operant additive summationas a reliable behavioral phenomenon, the ex-clusive use of positive reinforcement schedulesto maintain responding restricts its generality.Although Miller (1969) and Reiss (1969) dem-onstrated enhancement to compounded stim-uli of responding maintained by aversive con-trol, the independently conditioned stimulidid not explicitly control free-operant re-sponding. The present experiment investi-gated the effects of stimulus compoundingwhen stimuli independently controlling free-operant avoidance are compounded duringextinction and response maintenance.

METHOD

SubjectsThree naive male hooded rats, approxi-

mately 275 g at the commencement of train-ing, were maintained with free access to foodand water in their home cages.

ApparatusThe operant training chamber measured

8 in. high, 8.25 in. long, and 7 in. wide (20 by21 by 17.5 cm). The front and rear chamber

249

1972, 17, 249-256 NUMBER 2 (MARCH)

2HENRY H. EMURIAN and STANLEY J. WEISS

walls were constructed of aluminum, and theside walls and ceiling were 0.25 in. (0.6 cm)white translucent plastic; the ceiling wasvented with 0.25-in. (0.6-cm) diameter holes.The chamber floor was composed of 0.125 in.(0.3 cm) diameter stainless steel rods spacedapart 0.5 in. (1.3 cm) between centers. A Ger-brands microswitch lever was located on theright side of the front wall, 2.75 in. (6.9 cm)above the floor. A feeder trough, which wasinoperative throughout the experiment, wasmounted on the left side of the front wall atfloor level.A 2000-Hz tone stimulus employed in train-

ing and testing was generated by a BRSAA-201 audio oscillator, amplified by a BRSAO-201 amplifier, and presented through an8-in. (20-cm) speaker mounted within an en-closure centered 8.25 in. (20.6 cm) above thetraining chamber. The tone intensity was ap-proximately 90 dB measured at the leverwith a Type 1565-A General Radio SoundLevel Meter, scale C. With the exhaust fanoperating, the ambient noise level was ap-proximately 80 dB, and the tone stimulus waspractically inaudible outside the attenuationchest.The light stimulus employed in training

and testing was generated by two 6-in. (15-cm)25-w, 120-v bulbs, each horizontally mounted4 in. (10 cm) from the translucent side walls.Each bulb was enclosed in a ventilated clearplastic cylinder to augment heat dissipation.These two bulbs together produced approxi-mately 130.2 cd/M2 measured with a Honey-well Pentax 10/210 Photometer that was posi-tioned 5 in. (12.5 cm) from the chamber's sidewall.A shielded 7-w, 120-v bulb operating at 3 w

served as a houselight that was on continu-ously. The illumination it produced was toodim to activate the photometer, but it none-theless allowed the experimenter to view thesubject within the apparatus. The trainingchamber and its associated stimulus presenta-tion devices were enclosed within a sound at-tenuation chest described elsewhere (Weiss,1970). Solid-state scheduling equipment waslocated in a room adjacent to that housing thetraining chamber. Constant current shock wasgenerated by a Lehigh Valley Electronics 1531shocker and delivered to the grid floor, manip-ulandum, and front and rear walls through itsassociated scrambler.

ProcedureBaseline training. The performance of each

subject was stabilized on a multiple (mult)free-operant avoidance extinction (EXT)schedule. A free-operant avoidance schedule(Sidman, 1953) was operative during indepen-dent presentations of 3 to 3.5 min tone andlight stimuli. On this schedule, each leverpress postponed a shock for a constant periodof time, the response-shock (RS) interval. Inthe absence of responding, the time betweenshocks was determined by a shock-shock (SS)interval. The RS interval was in effect at eachstimulus onset. The avoidance stimuli wereseparated by shock-free intervals signalled bylight-out no-tone, T+L. These shock-free pe-riods varied from 3 to 5 min within sessionsand were followed immediately by a tone orlight stimulus. Tone and light stimuli werepresented in a nonsystematic order with thelimitation that no more than three identicalavoidance stimuli be presented in succession.Training sessions lasted from 4 to 5 hr.The lever was wired into the shock-duration

timing circuit to discourage the subject fromholding it down continuously. Respondingduring a shock reset the shock-duration timer,thereby prolonging the shock for the timeequal to the fixed shock duration. Response-shock and SS intervals, as well as shock inten-sity and duration, were adjusted across train-ing sessions to terminal values that yieldedstable avoidance and stimulus control for eachsubject. Shock intensity and duration for Sub-jects 6, 8, and 10 were respectively: 0.8 mA,0.5 sec; 2.2 mA, 0.5 sec; and 1.5 mA, 0.5 sec.The RS interval was 20 sec for all subjects,while the SS interval was 5 sec for Subjects 6and 10, and 2 sec for Subject 8.

Baseline training sessions continued untila subject satisfied a steady-state performancecriterion for five successive sessions of constantshock and schedule parameters. The criterionrequired (1) avoidance of at least 80% of thefirst shocks scheduled at each avoidance stim-ulus onset, tone. or light, and (2) abrupt andsustained response cessation during the shock-free T+L periods separating these stimuli.Subject 6 required 36 sessions to reach thecriterion; Subjects 8 and 10 required 24 and 23sessions, respectively.

Extinction stimulus-compounding test. Aftera 1-hr warmup on the training multiple sched-

250

COMPOUNDING STIMULI CONTROLLING AVOIDANCE

ule, a compounding test was administered withthe shock disconnected. One-minute tone,light, and tone-plus-light periods were pre-sented in each of 16 block replications. Thesestimuli, presented in nonsystematic order overreplications, were each separated by 3-minT+L interstimulus intervals. Subject 6 re-ceived all test replications in succession whileSubjects 8 and 10 received 15 to 20 min of thetraining multiple schedule, with the shock re-instated, between each four blocks of replica-tions.Maintenance stimulus-compounding tests.

After the extinction compounding test, thesubject received five consecutive sessions on itstraining multiple schedule. It was subse-

quently tested with the free-operant avoidanceschedule operative throughout all tone, light,and tone-plus-light stimulus presentations overthe following 28 sessions. Each session beganwith a 30-min warmup on the training multi-ple schedule followed by 3-min tone, light,and tone-plus-light stimuli presented once ineach of five replications. These stimuli wereseparated by 3-min shock-free interstimulusintervals signalled by T+L.

RESULTSTraining performance. The cumulative rec-

ords of eaclh subject, presented in Figure 1,graphically illustrate the stimulus control con-

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Fig. 1. Cumulative records of each subject from one of the last five training sessions. T and L denote toneand light stimuli delimited by the up position of the response pen. Each of these stimuli signalled a free-operantavoidance schedule. Light-out no-tone periods, during which the free-operant avoidance schedule was not in effect,are indicated by depression of the response pen. Oblique marks by the response pen indicate shocks.

251

HENRY H. EMURIAN and STANLEY J. WEISS

ditioned by the training schedule. Respondingprompted by the first scheduled shock occurredon fewer than 20% of stimulus_presentationsand essentially ceased during T+L periods.

Table 1

Means and standard deviations of responses and shocksper minute computed from the last five baselinetraining sessions.

Stimulus Conditions*

Responses Per Shocks PerMinute Minute

Subject T L T+L T L

Mean 12.4 13.3 1.1 0.5 0.56 SD 1.0 1.6 0.7 0.3 0.2

8 Mean 5.0 4.9 0.2 1.5 1.4SD 0.3 0.4 0.0 0.2 0.1

Mean 8.6 8.5 0.6 0.5 0.510 SD 0.8 0.5 0.2 0.2 0.0* T = tone, L = light, T+L= light-out no-tone.

The data presented in Table 1 indicate thatthe mean responses and shocks per minuteduring tone and light were essentially equiva-lent for each subject over the five criteriontraining sessions. The relatively high shockfrequency apparent in the terminal data ofSubject 8 suggests that the performance of this

Table 2

Total Stimulus-Compounding Test Responses

Stimulus Conditions*Subject Replications T+L T L T+L

1-4 97 41 46 05-8 52 30 6 0

6 9-12 35 15 3 013-16 35 7 3 0

Total 219 93 58 0

1-4 7 3 2 05-8 7 4 3 0

8 9-12 7 3 0 0.313-16 7 5 3 0

Total 28 15 8 0.3

1-4 19 14 8 0.75-8 22 4 5 0.7

10 9-12 21 8 4 0.313-16 28 8 5 0

Total 90 34 22 1.7

T+L = tone-plus-light, T = tone, L = light, T+L= light-out no-tone. T+L represents the mean of thethree 1-min pre-stimulus periods for each replicationtotaled within test quarters.

animal was controlled by shocks to a greaterextent than for Subjects 6 and 10.

Extinction test. Table 2 presents total re-sponse outputs of each subject during tone,light, and tone-plus-light within successive

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Fig. 2. Cumulative records of each subject for maintenance-test sessions one through four and each fourthsession thereafter. Successive ordinal positions of the five 3-min test-stimulus conditions represent the replicationsin which the stimulus was presented during respective test sessions. T+L = tone-plus-light, T = tone, L = light.A free-operant avoidance schedule was operative during T+L, T, and L. Oblique marks by the response pen in-dicate shocks.

252

COMPOUNDING STIMULI CONTROLLING A VOIDANCE

blocks of four test replications. All subjects ex-

hibited enhanced free-operant avoidance dur-ing the compounded stimuli throughout thetest: more responses were emitted during tone-plus-light than during tone or light. For Sub-jects 6 and 10, rate of responding declinedmore rapidly during the independent stimuli,causing the relative proportion of total re-

sponses emitted during the compounded stim-uli to increase as a function of successive rep-

lications.Maintenance tests. Figure 2 presents for

each subject cumulative records of each test

stimulus for Sessions 1 through 4 and eachfourth session thereafter. These records revealgraphically the tendency of the compoundedstimuli to control a response rate higher thaneither single stimulus condition throughout a

large proportion of the replications presented.A striking consistency noted in the perform-ance of all subjects is the lower shock fre-quency during the compounded relative to theindependent stimuli throughout the test ses-

sions.Figure 3 presents for each subject mean re-

sponses per minute and total shocks receivedduring each test stimulus over successive ses-

sions. Subjects 6, 8, and 10 maintained an

enhanced response rate to compounded as

compared to independent stimuli for 64.3%,60.7%, and 96.4% of the sessions, respectively.For Subject 6, the compounded stimuli main-tained enhanced rate throughout the first nineconsecutive sessions, after which rate over-

lapped with that maintained by the indepen-dent stimuli. For Subject 8, enhanced rate was

most evident during the latter half of the ses-

sions. Subject 10 exhibited enhanced rate dur-ing the compounded stimuli throughout al-most all maintenance-test sessions. Respondingduring T+L was negligible for all subjectsover sessions.

Figure 3 also indicates that the number ofshocks received by each subject during thecompounded stimuli was consistently less thanthose received during single stimulus condi-tions throughout successive sessions, even

when response rate was higher during the in-dependent stimuli. This suggests that (1) thehigher response rate occasionally observedduring tone or light was to some degree a func-tion of the greater number of shocks and con-

sequent post-shock response bursts inflatingrate during these stimuli, and (2) the com-

pounded stimuli maintained more efficientlyspaced responding, defined in terms of effec-tiveness in avoiding shock, than either tone orlight.

DISCUSSIONThe present experiment investigated the

effects of compounding stimuli that indepen-dently controlled free-operan avoidance re-sponding. When the stimuli re compoundedduring an extinction test, a' greater responserate was observed during the compoundedstimuli than during either single stimuluscondition. These response-enhancement effectsare similar to free-operant additive summationobserved when stimuli independently control-ling positively reinforced responding werecompounded, ano thereby extend the general-ity of free-operant additive summation to re-sponding maintained by aversive control.When the independently conditioned stim-

uli were compounded during response main-tenance, the compounded stimuli producedenhanced response rates for a majority of thetest sessions for two subjects, and enhancedresponding during 27 of the 28 sessions forthe third subject. For all subjects, fewer shockswere received during the compounded stimulithan during either single stimulus conditionthroughout the maintenance-test sessions. Ex-amination of maintenance-test cumulative rec-ords indicates that the high frequency ofshocks occurring during tone or light ofteninflated the response rate during those stimuli,thereby reducing the disparity in rate betweenthe compounded and independently condi-tioned stimuli. This suggests that the summa-tive effect during compounded stimuli mayhave been partially masked by the shock-inflated rates during the independent presen-tations of the stimuli composing it.Wolf (1963 Experiment 2) reported that en-

hanced responding during compounded stim-uli independently controlling respondingmaintained by VI schedules progressivelyabated as a function of exposure throughoutthree successive maintenance-test sessions dur-ing which the 4-min compound was presented36 times. With similar training schedules,Meltzer and Freeman (1971) reported en-hanced responding to compounded stimuliover sixty 1-min extinction probe tests. In thepresent experiment, the compounded stimuli

253

HENRY H. EMURIAN and STANLEY J. WEISS

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successive maintenance-test sessions. Legends are presented in the data frames of Subject 6.

254

COMPOUNDING STIMULI CONTROLLING AVOIDANCE 255

more effectively sustained control of rates ef-fective in avoiding shock than either tone orlight despite 320 min of contact with the com-pound during response maintenance. This sug-gests that summative effects might be bettersustained by stimuli associated with aversivethan positive contingencies.

Previous investigations reporting free-oper-ant additive summation employed multipleschedules during training in which respondingwas maintained during the independently con-ditioned stimuli by schedules of positive rein-forcement. With one exception (see Weiss,1971 Experiment 2) these studies employedextinction to control response cessation duringthe simultaneous absence of the stimuli main-taining responding. Within the context ofstimuli signalling positive reinforcement sched-ules, an extinction component, or timeoutfrom positive reinforcement, acquires aversivecharacteristics (Azrin and Holz, 1966). Thus,additive summation has been previously dem-onstrated after training in which the stimuluscondition controlling response cessation wasrelatively more aversive than the stimulus con-ditions maintaining responding.

In the present experiment, the stimuli main-taining responding signalled a free-operantavoidance schedule, while their simultaneousabsence signalled a period free from shock: atimeout from avoidance. Several experimentshave demonstrated that a timeout from avoid-ance acquires characteristics of a positive con-ditioned reinforcer (LoLordo, 1969; Verhave,1962), thereby indicating that this stimuluscondition in the present study was relativelyless aversive than the stimuli maintaining re-sponding. Therefore, the aversive relations be-tween those independently conditioned stimulimaintaining responding and their simultane-ous absence are reversed in the studies report-ing free-operant additive summation wherepositive reinforcement maintained responding,and the present study where responding wasmaintained by an avoidance schedule.

This comparison suggests that the absoluteaversive characteristics of the schedules main-taining responding during training is not alimiting condition in the production of free-operant additive summation. Additive sum-mation occurs with compounded stimuli thatare both relatively less aversive than theirsimultaneous absence, or relatively more aver-sive, depending upon the incentive controlling

responding-positive or negative reinforce-ment respectively.According to the analysis presented by Weiss

and Emurian (1970), the rate difference be-tween the stimuli maintaining responding andtheir simultaneous absence controlling re-sponse cessation was a necessary condition forthe summative effects observed in the presentexperiment. However, they also stated that thereinforcement densities during these stimuli". . . might interact with these rates to deter-mine the distribution of responses during astimulus compounding test" (p. 209). This in-teraction was supported by Weiss (1971, Ex-periment 2) who reported substantially weakeradditive summation when positive reinforce-ment was associated with response cessation,as well as the VI schedules maintaining re-sponding than when response cessation wascontrolled by extinction. Although this rate-reinforcement difference interaction was notinvestigated in the present study, the com-parison between stimulus compounding stud-ies employing positive reinforcement to main-tain responding and the present experimentthat employed an avoidance schedule clearlyindicates that a disparity in aversive character-istics between stimulus conditions existedwithin both classes of experiments. (We areusing the organism's preference, inferred frompast research cited above, to conceptualizeaversive relations between schedules effectiveduring the stimuli maintaining respondingand those controlling response cessation inboth incentive situations.) Therefore, it ap-pears that similar variables deserve attentionwhen predicting response distributions duringstimulus compounding when responding iscontrolled by either positive or negative con-tingencies.

REFERENCESAzrin, N. H. and Holz, W. C. Punishment. In W. K.

Honig, (Ed.), Operant behavior: areas of researchand application. New York: Appleton-Century-Crofts, 1966. Pp. 380-447.

Cornell, J. M. and Strub, H. A. A technique for dem-onstrating the inhibitory function of S-. Psycho-nomnic Science, 1965 3, 25-26.

LoLordo, V. M. Positive conditioned reinforcementfrom aversive situations. Psychological Bulletin,1969, 72, 193-203.

Meltzer, D. and Freeman, B. J. Maintenance of re-sponse summation under conditions of minimumstimulus intensity. Psychonomic Science, 1971 22,287-289.

256 HENRY H. EMURIAN and STANLEY J. WEISS

Miller, L. Stimulus compounding with an instrumen-tal avoidance response. Psychonomic Science, 1969,16, 46-47.

Miller, L. and Ackley, R. Summation of respondingmaintained by fixed-interval schedules. Journal ofthe Experimental Analysis of Behavior, 1970, 13,199-203.

Reiss, D. Pavlovian phenomena in conditioned accel-eration-stimulus summation. Conditional Reflex,1969, 4, 257-264.

Sidman, M. Avoidance conditioning with brief shockand no exteroceptive stimulus. Science, 1953, 118,157-158.

Verhave, T. The functional properties of a timeoutfronm an avoidance schedule. Journal of the Experi-mental Analysis of Behavior, 1962, 5, 391-422.

Weiss, S. J. Summation of response strengths instru-mentally conditioned to stimuli in different sensorymodalities. Journal of Experimental Psychology,1964, 68, 151-155.

Weiss, S. J. Free-operant compounding of low-ratestimuli. Paper presented at the 1968 convention of

the American Psychological Association, San Fran-cisco, California.

Weiss, S. J. Attentional processes along a compositestimulus continuum. Journal of ExperimentalPsychology, 1969, 82, 22-27.

Weiss, S. J. An effective and economical sound at-tenuation chamber. Journal of the ExperimentalAnalysis of Behavior, 1970, 13, 37-39.

Weiss, S. J. Free-operant summation and discrimina-tion training: consideration of non-reinforcementand response-differentiation consequences of So.Journal of the Experimental Analysis of Behavior,1971, 15, 387-402.

Weiss, S. J. and Emurian, H. H. Stimulus control dur-ing the summation of conditioned suppression.Journal of Experimental Psychology, 1970, 85, 204-209.

Wolf, M. M. Some effects of combined SDs. Journal ofthe Experimental Analysis of Behavior, 1963, 6,343-347.

Received 26 February 1971.(Final acceptance 8 November 1971.)